In the early days of telecommunication systems were relative simple compared to telecommunications today. A central office or telephone exchange was connected via 2-wire connection to telephone terminals. When a person wished to place a telephone call from one telephone terminal to another telephone terminal, a connection was established between the two terminals via the telephone exchange. Only AC signals, i.e. speech signals, were transmitted between the two terminals. A DC power source was connected directly to respective telephone terminals, usually this power source was a battery. The DC source is needed to get the telephones active. Ringing voltage for the bell was transmitted between the subscribers and the Central Office.
Later on the DC power source was moved from the individual telephone terminals to the network, e.g. central office or telephone exchange. A Battery Feeding DC Voltage, (VBAT at app. −48 Volts) was introduced at the 2-wire subscriber line by the exchange, placed at the central office. This new DC voltage at the 2-wire subscriber line replaced the batteries at the subscribers, and introduced new signaling possibilities. The possibilities included automatically on-hook and off-hook detection and automatic dial-pulse detection.
The injection of the DC on the 2-wire subscriber line was done in a way without damaging or corrupting the AC-signals, i.e. the speech signals, and most frequently by using Feeding Coils. Later on, other solutions were introduced, e.g. silicon solutions.
Increase in number of telephone terminals and connections have made the systems more complex as well as requiring more complex methods and apparatuses for testing connections. A DC-Feeding Bridge, also known as a DC-source, and a Holding Circuit, also known as a DC-load, are both suitable tools during transmission validation tests. A Holding Circuit is essentially the opposite of a DC-Feeding Bridge. Validation tests are typical carried out to document that a unit is compliant with a given specification.
A DC-Feeding Bridge is a common name for a circuit which is able to introduce a DC voltage on an analogue subscriber line or circuit without any significant degradation in the AC condition or performance of transmission line properties. For examples of using DC-Feeding Bridge and Holding Circuit during validation tests please refer to ETSI TR 101 953-1-1 section 6.3 and ITU-T G.992.3 section A.4.3.3.1.
A DC-Feeding Bridge which fulfils the guidelines or requirements given in ETSI TR 101 953-1-1 section 5 is not easy to design and realize. As mentioned in ITU-T G.992.3 section A.4.3.3.1, specific part of the report follows here, quote in italics:
The inductors and capacitors included in the set up need to be matched so as not to affect the results. When larger ratios of the impedance of the inductors and capacitors to the 50Ω resistors are used, less matching is required in these devices. Inductor matching is typically easier to achieve if a bifilar winding on a single core is used to create the matched pair. Adequate care should be taken to insure no resonance occurs within the measurement frequency range. This may require the use of two inductors in series (of different size) to meet this requirement when the measurement is broadband. It is also important to ensure that in tests that have DC current flowing, no saturation occurs in the inductors. It should also be noted that some types of capacitors vary in value with applied voltage, in general high quality plastic types should be suitable.
Resonance is more or less inevitable because the traditional implementation of a DC-Feeding Bridge requires two, or often more, inductors in series to achieve satisfactory results concerning impedance and bandwidth, to carry out ADSL2 broadband measurements. ITU-T G.992.3 deals with ADSL2, bandwidth 26 kHz to 1.1 MHz.
The problem increases with ADSL2+, bandwidth 26 kHz to 2.2 MHz, but the problem becomes enormous with the introduction of 6-band VDSL2, bandwidth 26 kHz to 30 MHz, because more inductors are needed.
One solution could be to divide the broadband measurements into two or even more different frequency bands, but both test time and complexity is contemplated to increase dramatically; this is, however, still the most common way to carry out the broadband measurements.